A ball check valve generally includes a highly finished metal ball that is biased against a conical seat by means of a compression spring. Pressure of fluid in the direction opposing the spring backs the ball away from the seat to enable flow through the valve, but pressure in the other direction supplements the spring force to bias the ball firmly against the seat and prevent flow.
Such ball check valves are widely used in grease fittings wherein grease is injected into a bearing, bushing or the like by means of a grease gun. There, the ball check functions to prevent outward flow of the grease. However, the cost of producing the highly finished metal ball check valves renders them rather expensive for use in many installations wherein cost is a significant factor.